1. Field of the Invention
The present invention relates to an underground power cable for transmitting electric power with being laid under the ground, and more particularly to an underground power cable having an optical fiber sensor for measuring distributed temperature of the cable.
2. Description of the Related Art
Generally, an underground power cable shows increase of temperature when electric current is supplied thereto, and an ampacity is calculated on the basis of a maximum temperature that the underground power cable may endure. Thus, while operating the underground power cable, it is very important to measure temperature of the underground power cable which is applying an electric current. However, since measuring the temperature of the conductor is substantially impossible while the underground power cable is applying an electric current, an optical fiber for measurement of temperature is generally used.
Korean Patent Laid-open Publication No. 2003-45864 discloses, as an example, a system for calculating temperature of a conductor of the underground power cable while a temperature measuring optical fiber is mounted in the cable.
In addition, there have been reported various power cable structures including an optical fiber in a power cable for temperature measurement or data transmission. For example, Japanese Patent Laid-open Publication No. 1990-144810 suggests a cable in which an optical fiber is arranged in a shielding layer or a cable jacket (or, a protective layer) of a power cable. Japanese Patent Laid-open Publication Nos. 1994-148001, 1994-181013, 1994-181014 and 1994-181015 disclose a cable in which an optical fiber is arranged in the shielding layer to a longitudinal direction.
U.S. Pat. No. 4,867,527 also discloses a composite power cable in which an optical duct containing an optical fiber is laid in a protective layer.
However, such conventional composite power cables have some technical problems as follow.
The optical fiber may be easily damaged since its mechanical strength is very weak. Thus, the optical fiber is received and protected in a tube (or, a pipe) having great strength and small radius of curvature. However, if the optical fiber is protected in the tube, the tube having small radius of curvature and great strength may damage an insulation layer, thereby deteriorating its insulating strength.
In addition, if the optical fiber or the optical duct is installed on an outer semi-conductive layer of the composite power cable, the optical fiber or duct is not fixed but movable in the power cable. In this case, the optical fiber or duct may be bent or inclined to one side more than its design optimum, so the temperature of a desired region cannot be accurately measured.